The present invention relates to electrochemical hydrogen storage alloys, rechargeable electrochemical cells, fuel cells using these alloys, and to methods of manufacturing the same.
More particularly, the invention relates to rechargeable cells, batteries, and fuel cells having at least one electrode formed of multicomponent, electrochemical hydrogen storage material or alloy. In one embodiment, such multicomponent alloys include discrete regions, small in size and widely distributed as will be hereinafter described, which differ compositionally from the bulk alloy and which contribute to the high rate capabilities of the alloys of the present invention. The present invention also includes methods of manufacturing the improved alloys to significantly further enhance such improved performance characteristics. The methods of the present invention assure that the size range and distribution of the aforementioned regions are optimized for enhancement of the rate capabilities of the alloys of the invention. Cells that incorporate these alloys have significantly improved performance characteristics, particularly with respect to exhibiting high discharge rates.
Rechargeable cells that use a nickel hydroxide positive electrode and a metal hydride forming hydrogen storage negative electrode (xe2x80x9cmetal hydride cellsxe2x80x9d) are known in the art.
When an electrical potential is applied between the electrolyte and a metal hydride electrode in a metal hydride electrochemical cell, the negative electrode material (M) is charged by the electrochemical absorption of hydrogen and the electrochemical evolution of a hydroxyl ion; upon discharge, the stored hydrogen is released to form a water molecule and evolve an electron:   M  -  H  +            e      -        ⁢                  ⟷        charge            discharge        ⁢    M    -  H  +      OH    -  
The reactions that take place at the positive electrode of a nickel metal hydride cell are also reversible. Most metal hydride cells use a nickel hydroxide positive electrode. The following charge and discharge reactions take place at a nickel hydroxide positive electrode:             Ni      ⁡              (        OH        )              2    -            OH      -        ⁢                  ⟷        charge            discharge        ⁢    NiOOH    +            H      2        ⁢    O    +      e    -  
In a metal hydride cell having a nickel hydroxide positive electrode and a hydrogen storage negative electrode, the electrodes are typically separated by a non-woven, felted, nylon or polypropylene separator. The electrolyte is usually an alkaline aqueous electrolyte, for example, 20 to 45 weight percent potassium hydroxide. The first hydrogen storage alloys to be investigated as battery electrode materials were TiNi and LaNi5. Many years were spent studying these simple binary intermetallics because they were known to have the proper hydrogen bond strength for use in electrochemical applications. Despite extensive efforts, however, researchers found these intermetallics to be extremely unstable and of marginal electrochemical value due to a variety of deleterious effects such as slow discharge, oxidation, corrosion, poor kinetics, and poor catalysis. These simple alloys for battery applications reflect the traditional bias of battery developers toward the use of single element couples of crystalline materials such as NiCd, NaS, LiMS, ZnBr, NiFe, NiZn, and Pb-acid. In order to improve the electrochemical properties of the binary intermetallics while maintaining the hydrogen storage efficiency, early workers began modifying TiNi and LaNi5 systems.
The modification of TiNi and LaNi5 was initiated by Stanford R. Ovshinsky at Energy Conversion Devices (ECD) of Troy, Mich. Ovshinsky and his team at ECD showed that reliance on simple, relatively pure compounds was a major shortcoming of the prior art. Prior work had determined that catalytic action depends on surface reactions at sites of irregularities in the crystal structure. Relatively pure compounds were found to have a relatively low density of hydrogen storage sites, and the type of sites available occurred accidently and were not designed into the bulk of the material. Thus, the efficiency of the storage of hydrogen and the subsequent release of hydrogen was determined to be substantially less than that which would be possible if a greater number and variety of active sites were available.
Ovshinsky had previously found that the number of surface sites could be increased significantly by making an amorphous film that resembled the surface of the desired relatively pure materials. One of the characteristics of hydrogen storage alloys, particularly electrochemically activated hydrogen storage alloys, which researchers have sought to improve is the hydriding-dehydriding kinetics, or the speed of hydrogen absorption and desorption. This affects the charge and discharge rates at which the battery can operate and determines the applications for which the battery is suitable. For example, such applications as hybrid electric vehicle propulsion require very high discharge rate capabilities in order to meet vehicle torque and acceleration requirements and also very high charge rates to accommodate regenerative braking requirements.
An approach to improving these kinetic characteristics has been to explore mixing alloys of the TiNi and LaNi5 types. TiNi alloys are often referred to as AB2 alloys and LaNi5 alloys as AB5 alloys.
Bououdina, et al. describe, in xe2x80x9cImproved Kinetics by the Multiphase Alloys Prepared from Laves Phases and LaNi5xe2x80x9d as published in Journal of Alloys and Compounds 288 (1999) 22-237, a new method to prepare hydrogen absorbing alloys from single phase intermetallic ones. To improve speed of hydrogen absorption and desorbtion, these authors intended to improve the hydriding-dehydriding kinetics, or speed, of the single phase system found in chromium and nickel modified Zirconium/Titanium Laves alloy (an AB2 type alloy). This was accomplished by melting of two single-phase intermetallic compounds. Of particular interest was addition of LaNi5 (an AB5 type alloy) to the described Laves AB2 alloy. This combination was chosen in light of the difficulties of preparing homogeneous alloys using rare earth elements and a Laves phase alloy, which stem from the approximately 750xc2x0 C. difference in melting temperatures. Another consideration was the ease of preparation of the LaNi5 as a single-phase material and its higher hydrogen kinetics coupled with a flat plateau for charging and discharging.
Yang et al. describe, in xe2x80x9cContribution of Rare-Earths to Activation Property of Zr-based Hydride Electrodesxe2x80x9d as published in the Journal of Alloys and Compounds, 293-295 (1999) pgs. 632-636, the effect of alloying cerium and either cerium-rich mischmetal or lanthanum-rich mischmetal on the crystalline characteristics and electrochemical performances of AB2 type hydride electrodes. Alloys of compositions ZrMn0.5Cr0.10V0.3Ni1.1 and Zr0.9T0.1Mn0.5Cr0.10V0.3Ni1.1 (T=Ml, mischmetal, or cerium and Ml=lanthanum-rich mischmetal) were prepared by arc melting under argon atmosphere with as-cast alloy ingots being crushed mechanically in air. Hydride electrodes were prepared by cold pressing the mixtures of different alloy powder with powdered copper in a weight ratio of 1:2 to form porous pellets of 10 mm diameter. Electrochemical charge-discharge tests were carried out in a trielectrode electrolysis cell in which the counter-electrode was nickel oxyhydroxide with excess capacity, the reference electrode was Hg/HgO.6M KOH, and the electrolyte was 6 M KOH solution. Discharge capacities were determined by galvanostatic method.
It was determined that cerium or mischmetal alloying can xe2x80x9cbasicallyxe2x80x9d solve the problem of activation of a ZrMn0.5Cr0.10V0.3Ni1.1 alloy electrode and that after rare-earth alloying, the maximum capacities rise from 250 mAh/g of the mother alloy ZrMn0.5Cr0.10V0.3Ni1.1 to 356 mAh/g of the cerium-containing one.
Yang et al. describe, in xe2x80x9cActivation of AB2 Type Zr-based Hydride Electrodesxe2x80x9d as published in ACTA METALLURGICA SINICA, Vol 11, No. 2 (April 1998) pgs. 107-110, the effect of lanthanum alloying on the crystalline characteristics, electrochemical capacity, and activity of AB2 type Zrxe2x80x94Crxe2x80x94Ni hydride electrodes. The alloys studied were prepared by arc melting under argon with the purity of the constituent metals above 99.9%. The as-cast alloy was crushed mechanically in air and sieved through 360 mesh. Hydride electrodes were prepared by cold pressing, with 300 mesh powdered electrolytic copper in a 1:2 weight ratio, to form porous 10 mm diameter pellets in copper holders. Electrochemical charge-discharge tests were carried out in a trielectrode electrolysis cell in which the counter-electrode was nickel oxyhydroxide with excess capacity, the reference electrode was Hg/HgO.6M KOH, and the electrolyte was 6 M KOH solution. Discharge capacities were determined by galvanostatic method.
The previously mentioned researchers found some level of incremental improvement in the hydriding/dehydriding kinetics of this combination of AB2 and AB5 metals. For the instant invention, however, we have found that development of short-range order, on the sub-micrometer to few micrometer level, will yield discreet regions of TiNi5-type alloys in a matrix of the TiNi-type alloy. While not wishing to be bound by theory, it is possible that there is some catalytic activity occurring at the surface of the TiNi5-type discreet region; it is possible that a synergistic effect relating to hydrogen catalysis or storage occurs at the TiNi-type matrix material boundary or interface with the TiNi5 region. Whatever is occurring, it is of notable beneficial use for electrochemical hydrogen catalysis, storage, reaction surfaces, and their combinations. While the mechanism of function here may not be understood clearly at the moment, we do know that with this combination of phases coupled with a rapid solidification, cooling, or quenching we obtain excellent electrochemical hydrogen storage with excellent electrochemical hydriding/dehydriding activity or, simply, excellent hydrogen kinetics and storage.
U.S. Pat. No. 5,554,456, issued to Ovshinsky, et al. Sept. 10, 1996, describes non-uniform heterogeneous powder particles for electrochemical uses, and said powder particles comprising at least two separate and distinct hydrogen storage alloys blended together. This invention notes the usefulness of magnesium-based alloys among others and describes methods of inclusion of multiple alloys in electrochemically activated hydrogen storage materials.
Appreciation of the Ovshinsky principles of disorder or ovonics requires that order and disorder be recognized at different levels particularly including those of composition, structure, and translation. Respectively, these levels of order or disorder may be considered on a distance scale of a few through several interatomic distances, through hundreds of interatomic distances, to generally more than a thousand interatomic distances. Again respectively, these varying scales of disorder will affect a material""s crystal structure or lack thereof, a material""s inter-grain boundaries or lack thereof, as well as a material""s morphology and surface characteristics. Application of these principles is useful in the field of energy storage but all levels of possible disorder must at least be recognized and dealt with to provide optimum overall properties. Some have dealt with different portions of the spectrum of ovonic principles, those of disorder, to advance the art. This invention recognizes the work of others and advances or builds upon those contributions to more fully realize the benefits of disordered ovonic materials by combining properties in a useful manner, and recognizing at least the translational level of disorder to gain benefit from such properties.
Ovshinsky describes, in xe2x80x9cAmorphous and Disordered Materialsxe2x80x94the Basis of new Industriesxe2x80x9d as published by The Materials Research Society in Materials Research Society Symposium Proceedings, vol. 554 (1999), pgs. 399-412, the principles of disorder and provides useful conceptual means for their understanding. Others have noted the usefulness of hydrogen absorptive materials and have worked with alloys of interest. Still others have recognized the usefulness of high hydrogen kinetics or rapid absorption/desorption of hydrogen, and worked with such materials. Beyond these researchers, others have combined two materials with differing characteristics. None, however, have arrived at the methods or means for producing the high capacities for rapid charging and high rate dependable discharging as well as stable storage capacity and high surface area as combined in the materials attainable through practice of our invention.
This invention provides, at least, electrochemical hydrogen storage materials with excellent storage capacity and excellent hydrogen kinetics. Such materials are very well suited to serve as electrodes for batteries, fuel cells, and in other applications in which high surface activity may be useful. Such products are obtained by combining alloys having good electrochemical hydriding/dehydriding characteristics with alloys having good electrochemical hydrogen storage capacity. These respective materials are the LaNi5-type materials or alloys and the TiNi-type alloys or materials; as noted earlier, these are also known in the art as, again respectively, AB5 and AB2 alloys or materials.
In particular, our invention provides multi-phase hydrogen storage materials suitable for use as electrode material for electrochemical reactions, energy generation, energy storage, combinations of these, and as reaction surfaces. This is accomplished by providing a combination of alloy phases in a unique microstructural arrangement in which at least one alloy having high hydrogen kinetics or storage characteristics is substantially evenly dispersed in minute, yet distinct, regions within a matrix of at least one alloy having good qualities in the complementary characteristic. This is accomplished by combining the at least two alloys, or their components, melting them and quenching rapidly to form the unique microstructure described. The rapid quenching should be accomplished in a manner to cause minute, discreet regions of one phase to form, yet also in a manner to prevent such minute regions from aggregating or growing too large.
This means that the quenching should be slow enough to allow precipitation to begin and some growth to occur; but rapid enough to prevent growth of overly large regions. Such a scheme may be determined without undue experimentation, particularly in light of the understanding that the benefit in this unique combination and microstructure apparently derives from a surface effect relating to the interaction of the distinct phases; benefit will accrue through optimization of that effect.
As a general matter small particles, or overall enhanced surface area, provide useful reaction surfaces. Smaller particles of this multi-phase alloy system will, in the absence of other competing factors, provide better electrodes or reaction surfaces. A method of enhancing the characteristics of interest for the storage alloys of this invention is gas-atomization. This method allows simultaneous provision of finely divided bulk multi-phase alloy as well as providing a means of attaining the rapid-quenching which is so useful in assuring, for example, the fine dispersion of the highly kinetic alloy phase within the high storage capacity alloy matrix phase in each discreet, atomized and solidified particle of the bulk combination.
Additional benefit appears to accrue by treating the quenched, particularly the atomized, powder in a manner to enhance exposure of the discreet regions of the finely dispersed phase. This may be accomplished by crushing, etching, cracking, and fracturing the particles or otherwise opening pathways for electrolyte media, in the exemplified alloy system, to reach the highly active surfaces or interfacial areas available throughout the individual particles of the alloy system.